1. Field of the Invention
The present invention relates to the field of gas turbine engines and is aimed in particular at a turbojet engine with a front fan.
2. Description of the Related Art
The rotor of the turbofan of an engine fitted to civilian aircraft comprises a disk driven by the low-pressure shaft and at the periphery of which a plurality of blades, extending axially, are held by their end that forms the root. In a commonplace type of structure the root of the blades is of dovetail cross section and these roots are housed in individual cavities machined in the rim of the disk in a substantially axial direction. Immediately downstream of the fan disk and making up the same rotor, is the boost compressor. This is in the form of a drum and comprises several blade stages.
The fan disk is secured to the drum of the boost compressor by being bolted to a radial flange thereof. The flange is also scalloped to form a means of axially retaining the blades of the fan disk. Each blade root, on its downstream side, is provided with an axial extension having two radial lateral slots and is engaged in a scallop in the flange of the boost compressor, as mentioned above, at the slots so as to be blocked against any axial movement.
In normal operation, the aerodynamic loadings on the fan blades have an axial component in the upstream direction. However, when the engine begins to windmill, the loadings on the blades reverse direction. Despite the aforementioned axial blockage there is a degree of play and the blades therefore move in the downstream direction. The part of the blade that forms the shank, between the dovetail root and the platform, then presses via its downstream transverse edge against the upstream flange of the drum of the boost compressor. This movement thus leads to friction causing wear which has been noted on the upstream face of the flange in the regions located in the continuation of the fan disk cavities.
Once this wear exceeds a certain depth it may have an impact on the life and integrity of the components in the event of the loss of a blade.
One problem associated with this phenomenon of wear stems from the presence, in these regions, of drillings which have been made to relieve the mechanical stresses in the flange. These stress-relief holes are generally closed by simple plastic plugs. The blade bearing against the flange in a region that has drillings is itself subject to erosion. Shank wear is therefore not uniform. The surface of the downstream face of the shank becomes worn only where it comes into contact with the flange; the surface portion that faces the drilling does not become worn and in the long-term becomes prominent. In addition to this phenomenon of blade wear, play develops between the blades and the flange.
Repair solutions have been proposed and these firstly consist in eliminating the worn zones by spot facing and then by reconstructing the reference surface facing the blade root. This latter operation uses plugs, known as anti-wear plugs, which are fitted tightly into the stress-relief holes of the drum.
The anti-wear plugs may be made of composite or of metal. The former has the advantage of wearing away far less rapidly than the latter. They are also more easy to remove when they need changing. By contrast, they do not have sufficient mechanical integrity when fitting the blades. For this reason, the anti-wear plugs are generally made of metal, but with a risk of damaging the drum during the fitting and removal operations that are needed to check the material health of the boost compressor drum stress-relief holes.
Specifically, the disadvantages are as follows:                There is a risk of creating defects when fitting the plugs.        Fitting the plugs is a tricky operation because the plug has to be a sliding fit in the orifice but a minimal shrinkage has to be observed in order to guarantee that it will remain in its housing during operation. This is achieved by a heating of the drum combined with a cooling of the plug. A significant temperature difference, in excess of 300° C., is needed.        Repairs are expensive requiring the machining of as many spot faces as there are fan blades, and an equivalent number of assembly operations.        Repairs have to be carried out at a specialist workshop which means down-time for the component, or even for the engine.        The drilling in the drum has to be rebored for each dismantling operation; that limits the number of replacements that can be done over the course of the life of the drum.        
As an at least partial solution to these drawbacks, the applicant company proposed improvements that make it easier to fit the plugs; for example in Patent Application FR 2 929 660 the plug is in two parts, a metal part on the upstream side that is subjected to the friction stresses and a plastic part on the downstream side making fitting and removal operations easier. In the Patent Application filed on 28 Jul. 2009 under the number FR 0 903 695, the plug is arranged in such a way that it can be fitted and removed just from the upstream side of the drum.
The applicant company has set itself the objective of further improving the anti-wear device so as to simplify the carrying-out of the repair operation, to reduce fleet production and maintenance costs and eliminate the risk of damage to the boost compressor drum when mount fitting the plugs.